1. Field of the Invention
The present invention is related to a hidden wideband antenna, and especially to a modified PIFA (planar inverted F-antenna) of which an open stub is provided to adjust electric matching and increase bandwidth.
2. Description of the Prior Art
A helix antenna of which the spiral coil is formed by winding metallic wires is the major type of antenna. Any of the followings including the diameter or the material of a coil or the pitch between rings of a coil or the total length of a coil of this kind of helix antenna will affect the set function in every aspect. But the defect of such conventional helix antenna resides in three-dimensional protruding out of the equipment. As for communication equipment of the modern miniaturized type or with necessary built-in antennas (such as a mobile phone or a portable computer), it can hardly be surely desirable.
Thus, various miniaturized and planar hidden microstrip antennas were gradually researched and developed. Among the modern applicable embodiments of the planar antennas, the relatively notable one is the planar inverted F-antenna (PIFA). In an early common planar inverted F-antenna as shown in FIG. 1, a metal wire 11 is provided on a grounding surface 10, a short point 12 is provided on one end of the metal wire 11, and a feed point 13 is provided near the short point 12, the feed point 13 is connected to a feed-in axle 14. A desired single-frequency antenna can be formed in this way. This early type inverted F-antenna can be developed to get a planar inverted F-antenna as shown in FIG. 2. Basically it includes a metal surface 15 of a predetermined area, and other related items including a grounding surface 100, a short point 120, a feed point 130 and a feed-in axle 140.
It was stated in xe2x80x9cDual-Frequency PIFAxe2x80x9d on page 1451 of xe2x80x9cIEEExe2x80x9d published in October of 1997 that, either to merge two separate blocks of different sizes into a rectangular shape or to directly provide an open slot with two mutually perpendicular sections on a rectangular metal surface can form a desired dual frequency PIFA.
Furthermore, a modified planar inverted F-antenna (PIFA) is researched and developed in recent years, which allows further diminishing of the PIFA size to make the antenna length be smaller than xe2x85x9 wavelength (xcex) and the antenna height be smaller than 0.01 wavelength (xcex), and to have a larger bandwidth (xe2x80x9cELECTRONICS LETTERSxe2x80x9d, Jan. 8, 1998, Vol.34 No.1). But the bandwidth is still low in application, and it is hard to be adapted to the modern hidden antennas of dual or multi-frequencies in a more desirable way.
The object of the present invention is to provide a hidden wideband antenna which is provided on a metal sheet with an open slot to partition it into a long section and a short section. The long and the short sections are both set to have a partial wavelength respectively of multi-resonant frequencies. An inwardly recessed section is provided at a specific position along the long section, and a bent back section is provided on the long section. The bent back section is provided with a protruding portion facing to the inwardly recessed portion and spaced therefrom with a predetermined distance in order to form an open stub to adjust frequency matching and increase bandwidth. This is the prime motive for the present invention.
The present invention will be apparent in its novelty and other features after reading the detailed description of the preferred embodiment thereof in reference to the accompanying drawings.